Any wireless network will broadcast its signals over a set frequency spectrum. In the case of narrowband networks, this will be a relatively short frequency spectrum. But the spectrum used for signals will increase as networks move to wideband and ultrawide bandwidth (UWB) broadcasting systems.
One problem that can arise in any network, but which is particularly troublesome in wider bandwidth systems is that of interfering signals. When a network broadcasts over a large spectrum there may be one or more narrowband interfering signals within that broadcast spectrum. Because of this interference, it may be desirable to limit the extent of transmission or reception over those interfering frequencies. In particular, on the reception side it may be desirable to avoid receiving the energy of interfering signals. While on the transmission side it may be desirable, or even mandated by law, to avoid transmitting signals that will interfere with certain narrowband networks.
By way of example, the current rules set forth by the Federal Communications Commission (FCC) allow for UWB networks to transmit in the spectrum from 3.1 to 10.6 GHz. This spectrum includes other signals (e.g., from cell-phone systems, radar, satellite links, altimeters, etc.).
One way to avoid the interfering signals is to include one or more notch filters in the receiver or the transmitter. These filters will reduce a frequency band from the transmitted or received signals, so that the energy transmitted or received over those bands is significantly lowered (depending upon the specific parameters of the notching filters used).
There are many types of notching filters that could be used, such as lumped filters (e.g., LC or twin-T), distributed filters (e.g., cross-coupled interdigital, open stub, or closed stub), or active filters (e.g., stable variable or salen and key), and their type and design are well known in the art.
The particular notching frequencies used for a given device may be constant or variable. For example, if there are known interfering signals that are likely to always be present, or for which transmission interference must always be avoided, a notching device may be pre-programmed to provide a frequency notch at that known notch frequency. However, for intermittent or local interfering signals, there may be no way to predict at what frequencies an interfering signal might be at. In this case it would be desirable to use a tunable notch and to provide a way by which a desired notching frequency should be dynamically chosen.